As is well known to those skilled in the art, various processes and catalysts exist for the homopolymerization or copolymerization of .alpha.-olefins. For example, processes are known for polymerizing ethylene or propylene, either alone or in the presence of small quantities of other monomers, to produce plastics. These plastics are typically used in such applications as blow and injection molding, extrusion coating, film and sheeting, pipe, wire and cable.
It is known that catalysts of a type generally described as Ziegler catalysts are useful for the polymerization of olefins under moderate conditions of temperature and pressure. It is also well known that the properties of polymer resin product obtained by polymerizing olefins in the presence of Ziegler-type catalysts vary greatly as a function of the monomers of choice, catalyst components, catalyst modifiers and a variety of other conditions which affect the catalytic polymerization process.
Important among properties of polymer resins is the average molecular weight and molecular weight distribution of the polymer. High molecular weights generally signify polymers having high strength properties. The melt index value ("MI") of a polymer is a measure of its average molecular weight. However two different polyethylene resins can have the same MI value and be significantly different in the distribution of the number of molecules of various molecular weights that make up the average molecular weight of the resin. Even though different resins may have the same MI value, one resin might have very different quantities of high molecular weight and low molecular weight fractions and thus exhibit very different processing characteristics and properties. Thus, the molecular weight distribution ("MWD") provides important additional information about the processability and mechanical properties of a polymer. The MWD may be determined by gel permeation chromatography measurements. Alternatively, another measure of the breadth of the MWD is Melt Index Ratio ("MIR").
Among Ziegler-type catalysts, supported vanadium catalysts are known for a tendency to produce polyethylene resins having a broad MWD. By reason of the resin's broad MWD a significant portion of the resin material may exist as polyethylene polymer chains having a low degree of polymerization, i.e. ethylene oligomers. As an example, European patent publication 099 660 teaches that unless certain precautions are taken, such ethylene oligomers are formed. The presence in the polyethylene resin of a significant proportion of low molecular weight oils or ethylene oligomers in the C.sub.10 -C.sub.32 range poses certain disadvantages to the use of such resin for blow molding applications. The ethylene oligomers have relatively low boiling points, and at elevated temperatures during processing of the polyethylene resin to form useful articles, such as blow molding of household or industrial containers, in which the resin is heated to about 400.degree. F. the ethylene oligomers in the resin vaporize, and, upon exposure to ambient air, condense into airborne droplets, making smoke. The smoke is considered objectionable by processors and impairs acceptability of broad MWD polyethylene resin products. Further, such low molecular weight oils can have other adverse effects. For example, since such materials have a waxy nature, they can exude into mold vent holes causing plugging of the hole and thus poor operation.
Among catalyst systems useful for thermoplastic polyolefin production are those utilizing a catalyst component in combination with an alumoxane cocatalyst. For example, U.S. Pat. No. 4,536,484 to Lacombe et al teaches a variation of the traditional Ziegler-Natta catalyst wherein the aluminum alkyl cocatalyst is replaced with the reaction product of an alumoxane and a magnesium composition. The magnesium composition is of the formula Y.sub.a Mg.sub.m X.sub.b wherein Y is an alkyl or alkoxy group having from 1 to 12 carbon atoms, X is a halogen, m is greater than or equal to 1 and a+b=2m. The reaction between the magnesium composition and alumoxane is carried out in an inert liquid medium. The resulting organo-magnesium-aluminum product is then subjected to chlorination and treatment with a transition metal compound wherein the transition metal is titanium, zirconium, vanadium, or chromium. Preferred transitional metal compounds include Ti(OR).sub.p Cl.sub.4-p) and Zr(OR).sub.p Cl.sub.(4-p) wherein R is a C.sub.1 -C.sub.8 alkyl radical and p is from 1 to 4. The catalyst may be supported on an inorganic or organic support and consequently may be used in gas or slurry phase reactors. The catalyst is said to be useful for the polymerization of C.sub.2 -C.sub.18 .alpha.-olefins, conjugated and non-conjugated dienes. The polymers produced using the Lacombe catalysts have melt flow ratios (MFR) in excess of about 30, indicating a broad molecular weight distribution.
The Chemical Abstract (105:173246z) of JP 61-141708 of Ube Industries relates to a solid catalyst system said to be useful for the production of a low halogen content polyethylene. One component of the catalyst system is a composition containing magnesium, a halide and a transition metal while the other component is an alumoxane. The first component is prepared by contacting a magnesium compound such as a magnesium halide, a hydroxy magnesium halide, a dialkoxy magnesium composition, etc. with a halogen containing compound such as aluminum, tin or silicon halides, alkoxysilane halides, etc. The product of this reaction is then treated with a transition metal composition preferably a titanium composition, such as TiX.sub.m (OR).sub.4-m wherein X is a halogen, R is C.sub.1 -C.sub.6 alkyl and m is 1 to 4; or P.sub.m TiX.sub.4-m wherein P is C.sub.4 -C.sub.6 alkyl; or vanadium halide or vanadium oxyhalide compounds.
The Chemical Abstract (107:96316j) of JP 62-072631 to Mitsui relates to a catalyst system for the production of dimers or oligomers of .alpha.-olefins. The catalyst system includes a transition metal component, a phosphorous ester/soluble magnesium component and, optionally, an alumoxane. The transition metal component may include bis-butoxytitanium dichloride, [Ti(OBu).sub.2 Cl.sub.2 ] tetrakis-butoxytitanium [Ti(OBu).sub.4 ], or tetrakis-2 ethylhexoxytitanium [Ti(O-2-ethylhexyl).sub.4 ].
Chemical Abstract (108:222271b) also lists Idemitsu Kosan's JP 63-003008 which relates to the production of polyolefins which are said to be of "high purity" and "high molecular weight." Idemitsu's process uses a catalyst system including oxygen-containing titanium compounds and the reaction product of water and organic aluminum compounds (apparently lo alumoxane compositions). The titanium compounds include those of the formulae: Ti(OR).sub.n X.sub.4-n, Ti(OR).sub.m (OR').sub.4-m, Ti(OR).sub.k (OCOR).sub.4-k or RO(Ti(OR).sub.2 O).sub.p -R wherein R and R' are C.sub.1 -C.sub.20 alkyl groups, X is a halogen, n is greater than 0 but less than or equal to 4, m and k are less than 4 but greater than 0, and p is greater than or equal to 2 but less than or equal to 20.
The Chemical Abstract (99:105887s) of Polish patent pL 116,247 discloses a supported transitional metal complex-alumoxane catalyst system for the production of polyethylene having a molecular weight of about 82,000 and a bulk density of 480 g/dm.sup.3. The transitional metal complex of the catalyst system is of the formula: Ti(OR).sub.n Cl.sub.3-n wherein n equals 0-3 and R is phenyl, C.sub.2 -C.sub.10 alkyl, chlorinated phenyl or chlorinated C.sub.2 -C.sub.10 alkyl.
Chemical Abstract (108:151044s) on an article by Oliva et al. in Makromol. Chem., Rapid Commun., 9(2), pp 51-5 (1988) relates to the production of polypropylene in the presence of soluble transition metal compounds and alumoxane as the catalyst system. The disclosure indicates that the solution process may employ tetrakis-butoxytitanium [Ti(OBu).sub.4 ], tetrakis-phenylzirconium [Zr(Ph).sub.4 ] or tetrakis phenyltitanium [Ti(Ph).sub.4 ] in combination with methylalumoxane as an effective catalyst system for propylene polymerization.
European Patent Application 241,560 describes a catalyst system the transition metal component of which is a hydrocarbyloxide compound. In that embodiment of the catalyst system comprising a tetrakis-hydrocarbyloxide transition metal compound (exemplified by propoxy and butoxy species of titanium and zirconium) or a bis-hydrocarbyloxide transition metal dihalide (exemplified by bis-phenoxy titanium dichloride) cocatalyzed by an alumoxane, the catalyst system is shown to polymerize propylene to an amorphous polypropylene of high molecular weight. In a second embodiment of the catalyst system, a transition metal halide is first reacted with an organic compound having at least two hydroxy groups to form a product compound wherein the two hydroxy groups of the organic compound are bonded to one transition metal atom. The product transition metal compound may be viewed as a bihydrocarbyloxide derivative wherein the hydrocarbyloxide ligands are interbridged one with another. Cocatalyzing this interbridged bihydrocarbyloxide transition metal compound with an alumoxane provides a catalyst system which polymerizes propylene to a crystalline polymer the isotactic stereoregularity of which depends upon the type of bihydroxy organic compound used to produce the transition metal derivative. Similarly, Miyatake et al., Makromol. Chem., Rapid commun., 10, pp. 349-352 (1989) describes the reaction of 2,2'-thiobis(6-t-butyl-4 methylphenol) to form titanium and zirconium derivatives having a bidentate ligand which, when cocatalyzed with methyalumoxane, provides a catalyst system which is active for ethylene and propylene polymerization.
From the above, it is clear that efforts have been directed toward the development of olefin polymerization wherein a transition metal complex is employed in combination with an alumoxane. Some of these transition metal complexes have included transition metals complexed with hydrocarbyloxide groups wherein the hydrocarbyl ligand is an alkyl or an aryl ligand. Those references which do indicate the MWD of the polymer product of the prior catalyst systems show a broad MWD of greater than about 30 MFR. It would be desirable to develop a catalyst system highly active for olefin polymerization which comprises a transition metal/alumoxane complex which allows the production of a polymer having both high molecular weight and a narrow MWD.